leukocytosis leu·ko·cy·to·sis or leu·co·cy·to·sis (lōō'kə-sī-tō'sĭs)n.pl.leu·ko·cy·to·ses (-sēz) An abnormally large increase in the number of white blood cells in the blood, often occurring during an acute infection or inflammation.

abnormally high number of white blood cells (leukocytes) in the blood circulation, defined as more than 10,000 leukocytes per cubic millimetre of blood. Leukocytosis is most commonly the result of infection. It may also occur after strenuous exercise, convulsions (e.g., epilepsy), emotional stress, anesthesia, the administration of epinephrine, pregnancy and labour, and lack of oxygen (as in the early phases of adaptation to high altitude). Leukocytosis is also observed in certain parasitic infestations, intoxications (metabolic or chemical), chronic diseases (e.g., leukemia), and allergic reactions.